The continuous development of fluid catalytic cracking (FCC) technology throughout the years has been propelled by several factors. Among these factors, the desire to produce petroleum products with superior quality and the fulfillment of the consumption demands are the principal ones. However, with the evolution of the requirements of environmental departments, FCC technology has evolved in the sense of fuel production that meets the environmental standards, as the contribution of the emissions of the gasoline-fueled vehicles represents a substantial part of the total emissions of pollutant compounds released in the atmosphere of urban areas.
Moreover, the development of the new clean burning engines and the introduction of the catalytic converters in the exhaust system of vehicles, from the 1980's on, obliged the companies that produce fuels to increase the quality standards, aiming at producing gasoline with higher octane and with low pollutant content such as, for example, sulfur and nitrogen. The future fuel demand outlooks indicate a reduction in gasoline consumption and a proportional increase in liquefied petroleum gas (LPG) consumption in the Brazilian market. It will be necessary to adapt the production units to this new reality.
In view of these trends, persons skilled in FCC technology continue to search for solutions that may provide flexibility to the processes, without losing sight of the product quality obtained and at the same time meeting the demands of the market. This trend is reflected in several filed patents that aim at meeting the above-mentioned objectives.
Several patents disclose recracking techniques with the objective of increasing the octane and changing the yield of FCC products. For example, U.S. Pat. No. 5,616,237 discloses a process where the stream to be cracked is divided into fractions and injected in the reaction zone in several points along the riser. The said patent also foresees the possibility of recycling the cracked products to the riser through the injection points.
Similarly, U.S. Pat. No. 5,154,818 discloses a hydrocarbon catalytic cracking process in multiple sections. In this patent, a light hydrocarbon stream is contacted with the catalyst used in the riser section, while a heavy hydrocarbon stream is contacted with the catalyst regenerated from another section of the riser, and a part of the effluent obtained in the first section is introduced in the second section of the riser.
In the inventions disclosed by the above-mentioned patents, the cracking of the hydrocarbon streams in the riser reduces the total capacity of the FCC unit, due to the introduction of a recycling stream in the reaction zone.
The spent FCC catalyst usage for converting the hydrocarbons is also presented in U.S. Pat. No. 6,162,402. In this patent, the spent catalyst is partially recycled for the reaction zone to favor the hydrocarbon yield. This approach also reduces the total capacity of the FCC unit.
The heating of the stripper in order to make possible a higher recovery and desorption of hydrocarbon steam of the catalyst surface is also another alternative. To reach a high temperature in the stripper, it can be adopted as heat transfer tubes or direct or indirect heat transfer from the catalyst that leaves the regenerator, as per U.S. Pat. No. 5,234,578. The stripper temperature is a function of the new reaction temperature increase. Consequently, the increase of the temperature in the stripper implies increased reaction temperature and severity of the process. Thus, the temperature increase can cause a negative effect in the profile of the obtained products and increase the power consumption of the unit.
U.S. Pat. No. 6,238,548 claims the recycling of naphtha for the region of dense phase, located at the top of the stripper, with gain in the octane. The injection in this region reduces the contact between the naphtha and the catalyst, reducing the conversion and the improvement of quality of the FCC products.
In view of the above, a solution is desired that does not reduce the capacity and the unit conversion, as well as the improvement of FCC product quality. In addition, a solution is desired with the possibility of a selective cracking on the stripping vessel, in order to optimize the yield of relevant products without compromising the production in the reaction zone (riser).